1. Field of the Invention
The present invention relates to a gas sensor used for measuring a concentration of a specific gas component, especially hydrocarbons, in an exhaust gas generated from, for example, automobiles, a method of measuring a gas component by utilizing such a gas sensor, and a method of diagnosing a malfunction of an exhaust gas purifying apparatus.
2. Related Art Statement
Recently, there is a requirement that a malfunction of an exhaust gas purifying apparatus provided at a downstream position of an automobile engine be detected and reported to the driver of the automobile. This kind of malfunction can be detected by measuring an increase of hydrocarbon concentration in an exhaust gas passing through the exhaust gas purifying apparatus. In order to limit such an increase of hydrocarbon concentration, there is OBDII in California, in which a total amount of NMHC (non methane hydrocarbon) is limited under a predetermined level. Therefore, there is a necessity for measuring a concentration of hydrocarbon in the exhaust gas of automobiles.
Usually, as a gas sensor for measuring a concentration of hydrocarbon in a gas to be measured, a total hydrocarbon analyzer utilizing a hydrogen flame ionization method is known. This total hydrocarbon analyzer measures a hydrocarbon concentration by supplying hydrocarbons into a hydrogen flame to perform an ionization, measuring such an ionization as a small ion current by applying DC voltage between electrodes which sandwich the hydrogen flame, and calculating the hydrogen concentration from the thus measured small ion current.
In the total hydrocarbon analyzer for measuring an HC (hydrocarbon) concentration mentioned above, it is possible to measure accurately the HC concentration in the exhaust gas. However, in the total hydrocarbon analyzer utilizing a hydrogen flame ionization method, it is necessary to use hydrogen, and also it is necessary to use a large and complicated mechanism. As a result, it is not possible to construct a compact sensor which can be used in automobiles by the total hydrocarbon analyzer.